The present invention relates generally to vehicle tire pressure maintenance, and more specifically, to tire pressure maintenance devices contained on a wheel of a vehicle that automatically regulate and maintain a desired tire inflation pressure or amount of air in a tire.
Under-inflation of vehicle tires is dangerous, deadly, and common. Under-inflation is involved in hundreds of thousands of accidents, tens of thousands of injuries, and hundreds of fatalities annually in the U.S. During 2000, a large number of SUV rollovers and deaths were attributed to significantly under-inflated tires, bringing significant attention to the problem. In hope of reducing the unacceptably high rate of accidents, injuries, and deaths related to under-inflation, the United States Congress passed the TREAD Act of 2000 that requires a warning system in new motor vehicles to indicate to the operator when a tire is significantly underinflated. Consequently, the National Highway Traffic Safety Administration (NHTSA) proposed a safety standard requiring that as of 2007, all new passenger cars, trucks, multipurpose passenger vehicles, or busses under 10,000 pounds must be equipped with a tire pressure monitoring system (TPMS) to warn a driver when any tire is under-inflated by 25% or more. The program is estimated to cost well over $1 billion annually.
However, even if the controversial TPMS program achieves its estimates it will reduce under-inflation related accidents by about 20%. Many industry experts doubt that it will help at all. A device that automatically maintains proper tire inflation will eliminate almost all accidents, injuries, and deaths due to under-inflation. In addition, an effective tire pressure maintenance device will improve fuel efficiency by about 2% and will reduce tire tread wear by about 10%, more than paying for the devices and saving many billions of dollars annually in the U.S. if in widespread use.
The temperature of air in a tire has a major effect on the pressure of air in the tire that must be considered in any approach to tire pressure maintenance. FIG. 1 shows how tire pressure varies with temperature according to the ideal gas law. The four pressure-temperature (PT) lines illustrate the pressure-temperature behavior of a tire filled to 32 psi at air temperatures of 20, 40, 60, and 80° F., assuming a constant tire volume. The four PT lines represent four different amounts of air in the tire. Ambient temperature variations and tire heating from rolling make tire temperatures and pressures move up and down along the PT line denoting the amount of air in the tire. A tire will move to a higher PT line only when air is added and to a lower line only when air is released or leaks out of the tire.
As shown in FIG. 1, the pressure in a tire increases and decreases about 1 psi with temperature increases and decreases of 10° F. Normally, as a car is-driven the temperature in the tire increases about 2 to 5 psi above its “cold” pressure (at ambient temperature) due to the heat caused by flexing of the side-walls and friction from road contact. The recommended manual tire inflation procedure is to fill tires monthly to the manufacturer's recommended cold pressure (MRCP or “placard” pressure) at ambient temperature. In practice, tires are usually filled less often and while warm from driving. An ambient temperature drop of 50° F., possible within a day and common within a month, reduces tire pressure by about 5 psi. Thus, tire pressures frequently fall 8 psi below the MRCP, typically 25%, without considering the normal leak rate of about 1 psi per month. Most under-inflation is due to inadequate manual tire pressure maintenance and it is killing and maiming people at an unacceptable rate.
Two approaches to automatic tire pressure maintenance goals are:    1) Constant Pressure—maintains the MRCP independent of temperature by adding air when the warm tire pressure is below its warm objective (about 3 psi above the MRCP); and    2) Constant Amount of Air—maintains the amount of air in the tire that produces the MRCP at a selected temperature by adding air any time the tire temperature and pressure fall below the related PT line.
Both approaches replace air that leaks from tires and assure less variation from the MRCP than manual inflation procedures, with or without a TPMS. The constant amount of air approach will minimize deviations from the PT line due to temperature changes and will minimize the amount of air pumped into a tire to maintain the desired inflation pressure.
Many patents have been granted on approaches to automatically maintain the desired inflation pressure in pneumatic tires. None address temperature variation significantly. Of the related approaches, one involves a difficult generation of two continuous out of phase A.C. voltages that are rectified to provide a continuous D.C. power source for a D.C. motor-driven air compressor on the wheel. Another discloses a battery operated compressor contained on a wheel with no practical means for recharging the battery. Another requires a TPMS or an on-wheel pressure sensor to send low tire pressure data from the wheel to the vehicle body in order to activate an electromagnet that drives a compressor on the wheel. However, none have produced a practical device. Therefore, there are needs in the art for a tire pressure maintenance means that:    automatically maintains proper tire inflation without operator attention or maintenance;    is small, simple, practical, inexpensive and that provides long term reliable operation;    is self-contained on a wheel assembly and operated by wheel rotation;    is fail safe such that failures do not cause deflation or over-inflation of a tire;    alerts drivers to excessive tire leaks or failures of the pressure maintenance means; and    provides a higher emergency inflation rate to mitigate leak rates and increase the time for drivers to reach a safe place.